Production plants, such as air separation plants, supply gaseous products to customers by means of pipeline systems. Typically, demand patterns for each customer tend not to be constant so that the total demand of all customers for the gaseous product will range above and below production capability of the production plant or plants supplying the gaseous product.
In order to meet customer demand for the gaseous product under circumstances in which the total demand is above production capability, the pipeline system used to supply the customers is operated with, what is known in the art, as “pressure pack”. In pressure pack, pipeline pressure or other storage capacity pressure is increased during periods of low demand and additional gas is supplied to the customers by reducing the pressure during periods of high demand. Additionally, during high demand periods, stored liquid may be vaporized to meet the demand.
The volume within the pipeline system necessary to store sufficient quantities of gas is either provided by the pipeline itself or through the use of gas receivers which are nothing more than high pressure storage tanks. Economic inefficiencies creep into such supply schemes through gas venting and excessive vaporization of liquid. For instance, when the pressure within available gaseous storage is too high, the gaseous product is vented to maintain safety limits. On the other hand, when pressure is too low, the additional gaseous product supplied by vaporization of stored liquid is expensive in that the liquid, as compared to the gas, is a value added product due to the expense involved in liquefying the gas.
It is therefore important to control the production of the air separation plants in the pipeline system to minimize the vaporization of liquid product and the venting of gaseous product. One solution to minimize the venting of gaseous product or the vaporization of the liquid product is to set the total production equal to the average customer demand over a given time period. Another possible solution is to match the production with customer demand as closely as possible. The forgoing solutions are not, however, practical or even possible to implement. A major reason for such impracticability is that instantaneous customer demand used in forming an average or matching production is often not readily available. Additionally, the customer usage pattern can be unpredictable and the demand itself can change so rapidly that air separation plants and other production plants are not able to change production in a sufficiently short period to match the customer demand. Even in a supply situation in which the customer demand changed at a rate that was sufficiently slow that plant production levels could match the change in customer demand, the resulting plant upsets would lead to seriously reduced product yields.
As a result, plant production levels are manually set by a plant operator who bases plant production on storage pressures, changes in such pressures, perceived customer demand patterns, and experience as to the characteristics of plant response upon control inputs and the rapidity at which pressure within the pipeline or gaseous storage reacts to changes on both the supply and demand side.
As may therefore may be appreciated, the control of one or more air separation plants or other production plants supplying a pipeline lends itself to an automated supervisory control system to set production targets. For instance, U.S. patent application Ser. No. 20030144766 utilizes model predictive control for controlling the target levels of air separation plants that supply a gaseous product to a pipeline. The target levels for the air separation plants are optimized to maintain pipeline pressure within a predetermined range. The optimization is based upon predicting an open loop responsive pressure and to formulate production request changes that are required to at least in part to restore the pressure to the target value from the open loop response. Such a control method is very useful for the control of large pipeline distribution networks. The implementation of such a control method in smaller networks may not always be cost effective in that extensive testing is required to implement such a system.
As will be discussed, the present invention, which is amenable to both large and small pipeline distribution networks, utilizes fuzzy logic control to control production to closely match demand of the customers and thereby to minimize the need to either vent gaseous product or vaporize liquid product.